The object of the present invention is an improved flexible bar structure for spectacles. It is known that spectacles bars should be flexible enough to be elastically opened wide apart, to an extent sufficient to allow the spectacles to be easily put on, while--on the other hand--they should be stiff enough to produce a correct contact pressure on the temples.
This flexibility of the bars is at present obtained with different techniques. Amongst them, the simplest, most diffused and most economic one is no doubt that of producing the bars of a plastic material having elasticity properties of its own, suited for the purpose.
Nevertheless, the drawback of this technique is that the characteristics of flexibility of the bars are inadequate. In fact, if the plastic material from which they are made is very flexible, it will no doubt be easy to put on the spectacles, in that the bars can be opened wide apart, but the contact pressure of the bars on the temples is too weak to guarantee a proper grip of the spectacles in a position of use. If, viceversa, the plastic material is quite stiff, it will be more difficult to put on the spectacles and, on the other hand, the pressure of the bars on the temples may prove uncomfortable after the spectacles have been worn for some time.
Another technique is that normally used for spectacles frames made entirely of metal. In this case, the bars are formed as relatively stiff metal stems, anchored to the front part of the frame by means of so-called elastic hinges. An elastic hinge is preferably configured so as to have a dead center position--or position of stable equilibrium--in which it is returned by suitably calibrated spring means. The combination of these two elements--namely, the spring-loaded hinge and the relatively stiff metal bars--allows both to easily put on the spectacles, in that the elastic hinges allow to open the bars wide apart, and to comfortably wear them, in that the stiff bars, under the return action of the springs of the hinges, produce a correct contact pressure on the temples. Nevertheless this technique is, as already said, on one hand almost exclusively limited to entirely metallic frames, and this for manufacturing purposes, and on the other hand it is fairly expensive for the use of the abovementioned type of hinge.
To improve the characteristics of flexibility of plastic bars, it has been proposed to insert into the body of the bar a flexible core in the form of a thin metal lamina. The improvement thereby obtained essentially derives from the fact that a bar thus made can undergo, with heat, a permanent deformation, so as to be adapted in a more precise manner to the shape of the head of the person wearing the spectacles; nevertheless, its characteristics of flexibility are not substantially improved.
A further proposal is provided by the FR-A-1.238.393 or by the EP-A1-0.061.555, and it consists in forming the bar in two parts and connecting them by means of a cylindrical spiral spring. Also this solution has not been found satisfactory because said spiral spring, on one hand, provides a flexibility in every direction (see FIGS. 1 and 2 of FR-A-1.238.393)--which constitutes a drawback as far as wearability and comfort of the spectacles--and, on the other hand, it tends to loose efficiency and slacken with time. The drawbacks of these known types of spectacles bars have been pointed out in the FR-A-2.588.389, which proposes--in order to overcome the same--a bar structure comprising a metal stiffening core onto which a spiral spring is fitted. According to what is expressly stated in this patent, the stiffening core is apt to be deformed so as to be adapted to the person wearing the spectacles, while the elasticity is guaranteed by the spiral spring. In reality, experience has proved that in this bar structure the stiffening core, in order to be able to preserve the desired shape, obtained by non-elastic deformation, has a rigidity such that the spiral spring fitted thereon is no longer apt to guarantee any elastic action.
Furthermore, according to the manufacturing method of this patent, the stiffening core is fixed to the bar portion carrying the hinge and, onto the same, there is first fitted the spiral spring and subsequently the second part of the plastic bar. This first of all involves considerable manufacturing difficulties when having to fit the second plastic part of the bar, which operation has to be carried out manually, and furthermore, it also involves problems as far as a steady fixing of said second part: experience teaches that, in a bar thus formed, the second plastic part can easily fall apart by slipping out during use. A further drawback the lies in the fact that--as in the case of FR-A-1.238.393--this structure provides a flexibility in every direction; and this is all the more serious in that the spring is allowed to rotate over the core, since nothing checks it, and can thus damage the ends of the plastic parts.
Substantially the same drawbacks occur with a bar structure as that described in the DE-C-684.189, which is besides relatively complex and difficult to manufacture, especially due to the presence of a tubular core.